This disclosure generally relates to control of xerographic marking engines, such as copiers and laser printers.
The basic xerographic process used in a xerographic imaging device generally involves an initial step of charging a photoconductive member to a substantially uniform potential, Vcharge. The charged surface of the photoconductive member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas irradiated by the light image. This procedure records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the document being produced. The latent image is then developed by bringing a developer material including toner particles adhering triboelectrically to carrier granules into contact with the latent image. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet having the toner image thereon is then advanced to a fusing station for permanently affixing the toner image to the copy sheet in an image configuration.
Control of the initial field strength, Vcharge, and uniformity of the charge on the photoconductive member is very important because consistently high-quality reproductions are best produced when a uniform charge having a predetermined magnitude is obtained on the photoconductive member. In discharge area development, if the photoconductive member is overcharged, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of development material will be correspondingly decreased. As a result, the copy produced by an overcharged photoconductor will be faded. Moreover, if the photoconductive member is excessively overcharged, the photoconductive member can become permanently damaged. If, however, the photoconductive member is not charged to a sufficient level, too much developer material will be deposited on the photoconductive member. The copy produced by an undercharged photoconductor will have a gray or dark background instead of the white background of the copy paper. In addition, areas intended to be gray will be black and tone reproduction will be poor.
The life of the photoconductor in a xerographic marking engine, is typically limited by the occurrence of some form of print quality defect related to the photoconductor. One of the typical failure mechanisms is the slow wearing away of the surface layer of the photoconductor. Eventually, after enough of the surface layer has been worn away, print quality defects begin to appear in the image prints generated using the worn photoconductor. An example of this type of defect is the charge deficient spots (CDS) defect that appears in some print engines after approximately 10-12 um of the photoconductor outer layer, i.e., the charge transport layer (CTL) has been worn away.
Since photoconductors are typically somewhat expensive to replace, the life of a print engine's photoconductor can have a significant impact on the overall operational costs of the print engine.